1. Field of the Invention
This invention relates to aqueous suspensions of magnesium hydroxide. More particularly, it is concerned with suspensions having satisfactory fluidity at a high concentration coupled with low tendency towards sedimentation.
2. Description of the Prior Art
Magnesium hydroxide is widely used in the chemical industries as the starting material for the preparation of magnesium sulfite in the magnefite process, as a neutralizing agent, as a gas absorber and in other utilities. It has recently attracted attention as a sulfurous acid gas absorber in flue gas as well as an additive to boilers using heavy oil for prevention of both high and low temperature corrosion. Magnesium hydroxide is transformed in the form of a slurry, cake or dry powder. A disadvantage of aqueous slurries is that the solid content is 35 percent at the highest. This markedly increases the transportation cost. In addition, there is a high tendency towards sedimentation or separation during the transportation. The form, therefore, is unsuitable for transportation. If the solid content of magnesium hydroxide is increased to over 35 percent the viscosity is 10,000 cp. or higher so that a cake with no fluidity is formed. Although transportation in this form may be possible, the handling of the cake is very inconvenient. Moreover, the reversion of the cake to a slurry, which is necessary for practical uses of magnesium hydroxide, requires a large amount of power and time due to the high viscosity of the cake.
Alternatively, magnesium hydroxide is often transported in dry powder. This is not satisfactory, however, since even if finely pulverized, the particles do not completely recover their original size. Moreover, aggregation of the particles and reduction in the surface activity will result in a higher tendency towards sedimentation and lower reactivity.
It is therefore desirable to transport magnesium hydroxide in slurry form, but with a concentration comparable to the cake, but with good fluidity and decreased sedimentation tendency.
Attempts which have been made by prior art techniques to prepare aqueous suspensions with high concentration of magnesium hydroxide have not been successful. Consequently, means have been adopted to transport magnesium hydroxide in organic oily preparations at a high concentration by the addition of emulsifiers, surface active agents or protective colloid forming materials (cf. Japanese Pat. Publications 9581/63 and 31863/70). In these cases, however, special uses such as for the fuel additive are only possible because of the extender being an organic oily substance. Moreover, these preparations are expensive.
The magnesium hydroxide that is obtained by reacting lightly calcined dolomite with salt water containing magnesium halogenides and calcium chloride as the mother liquor is known to yield fluid aqueous suspensions of high concentration (cf. U.S. Pat. No. 3,451,774). This macrocrystalline magnesium hydroxide prepared by this special method has a lower water content, good fluidity of the suspension, and ready filterability, but readily forms sediment.
Colloidal magnesium hydroxide with low tendency of sedimentation can be produced under specialized reaction conditions, but it is difficult to filter and the solid concentration is low.